Named by descendants of those held in Chinese internment camps before the Great War, yao guai are mutatedblack bears and one of the more vicious creatures of the wasteland, sporting razor sharp claws and teeth and a nasty temperament. Despite this, yao guai exhibit a gentle, familial bond toward one another when found together, and can even be domesticated.
Yao guai will attack nearly any other animals or people they encounter, including deathclaws. The mutation has greatly extended their lifespan, although practical immortality has only been achieved by ghoulified yao guai, limited to remote populations on the eastern seaboard in Point Lookout and on Mount Desert Island. Some of these yao guai are bears that have been born before the Great War.
The yao guai of Zion Canyon are visually identical to the yao guai of the Capital Wasteland, but are typically larger. In Zion Canyon, they form small dens of three to four yao guai and live in the multiple caves the pocket the canyon. They exhibit a gentle, familial bond toward those in their dens, provided they do not feel threatened by an observer's presence. Yao guai will attack nearly any other animals or people they encounter. They apparently subsist only on the geckos that inhabit Zion, as they do not attack bighorners in the same area. This may appear uncharacteristic, but is consistent with predators not unnecessarily exhausting their food supply.
Yao guai are extremely fast and powerful creatures, second only to deathclaws in terms of raw strength and speed. Unlike deathclaws, they are less alert; the player may come in contact with one when it is asleep or laying down. Unlike in Fallout 3, yao guai are not affected by the Animal Friend perk and will attack on sight. The Sorrows tribe has been able to use yao guai paws to create yao guai gauntlets. These gauntlets use the weight of the bear's paw and its sharp claws to deliver a vicious attack.
These creatures are nearly identical to the yao guai found in the Capital Wasteland. They are second to only the deathclaw in terms of raw strength and speed. They are also normally found in groups of two to four.
Giant yao guai are larger versions of yao guai. A giant yao guai can easily kill a low-level player if they're not careful. Early in the Honest Hearts add-on, a giant yao guai attacks and kills a green gecko on top of a rock while the player approaches the Dead Horses Camp. A few of these can be found near The Red Gate.
In Chinese, yao guai (妖怪) can not be simply captured in English; the variety range of words that can represent the meaning include monster, ghost, spook, sprite, giant, undead, devil, demon, fiend, evil spirit, elf, goblin, boggy, and fairy.
In Chinese mythology, "yao guai" are physical manifestations of the spirits of mistreated animals, or fallen divine animals.
↑Fallout 4 loading screen hints: "Originally named by the descendants of those held in Chinese internment camps before the Great War, the Yao Guai is a ferocious mutated bear."
↑Quercus: "Oh, how wonderful! It's a real yao guai, descended from the American black bear. "